Oct 24 (Reuters) - Biogen Inc beat Wall Street estimates for third-quarter profit as it curbed spending, but U.S. sales of Spinraza, a potential future blockbuster drug, were disappointing and sent the U.S. drugmaker's shares nearly 3 percent lower on Tuesday.

Spinraza, Biogen's spinal muscular atrophy (SMA) drug, generated $197.6 million in U.S. sales for the third quarter, lagging analysts' average expectation of $242 million, according to broker SunTrust Robinson Humphrey.

"Spinraza performance in the U.S. seems to be impacted by challenges with the route of administration, though ex-U.S. sales were solid," SunTrust Robinson Humphrey analyst Yatin Suneja said.

About two-thirds of SMA patients have the Type 2 or Type 3 form of the disease, and some 60 percent of Type 2 patients might have had spinal surgery, which creates challenges in administering Spinraza to them, and potentially slows uptake in these patients, analysts have said.

The U.S. Food and Drug Administration approved Spinraza last year to treat SMA, a rare disorder that is the leading genetic cause of death in infants.

Spinraza costs $750,000 for the first year of its use and drops to $375,000 a year later.

Total revenue climbed 4.1 percent to $3.08 billion, helped by strong international sales of Spinraza.

Biogen's shares, up nearly 26 percent since the beginning of the year, were trading 2.6 percent lower at $319.99 before the bell. (Reporting by Akankshita Mukhopadhyay and Manas Mishra in Bengaluru; Editing by Martina D'Couto and Sai Sachin Ravikumar)